Lamp dimmers for coupling to the AC mains supply voltage typically employ angle modulation of a switching device such as a triac so as to adjust the duty cycle of the AC dimmer output signal. In so-called “leading edge” dimmers, the triac is selectively operated to adjust the duty cycle (i.e. modulate the phase angle) of the dimmer output signal by removing rising portions of AC voltage half-cycles (i.e. after zero-crossings and before peaks). In so-called “trailing edge” a triac can be controlled to remove falling portions of AC voltage half-cycles (i.e. after peaks and before zero-crossings).
With the growing popularity of LEDs for domestic and other lighting, the need to adjust the brightness of LEDs is increasing. Since AC lamp dimmers are commonly available, it would clearly be desirable to allow them to be used also for LEDs. However, there are several technical reasons which militate against this. One problem is that LEDs are powered using DC typical converters and are not ideally suited to operation from an AC supply particularly when operated at reduced output. Specifically, when power is reduced such that there is insufficient load on the triac, this gives rise to flicker. This is unpleasant when dimmers are used with low power halogen lamps that may have a power rating of 20 W, but it can be quite intolerable when used with LEDs having a power rating of only 1 W.
Typically converters used for AC-operated lamps, such as halogen lamps, are based on the conversion of low frequency mains voltage AC to high frequency, low voltage AC. The voltage that is applied to the lamp is the low frequency envelope that contains high frequency harmonics, which are undesirable when using LEDs.
Also with conventional converters used with halogen lamps, when there is no dimming there is almost unity power factor. But when dimming is used, the power factor may fall to as low 0.3. As opposed to this, converters for use with LEDs are based on a different topology, which employ power factor correction so as ensure that the power factor does not fall below 0.8 when dimming occurs.
U.S. Pat. No. 6,304,464 discloses a circuit arrangement for operating a LED array with an installed power in the range from 6 W as a minimum to at least 15 W. A flyback converter is used to achieve good power factor as well as a low level of harmonic distortion (THD) of mains current extracted from the supply source.
Further since the mains voltage is subject to fluctuations typically in the order of ±10%, the output of the converter is likewise subject to the same fluctuations. This also is unsuitable for use with LEDs, which require a stabilized voltage source.
Power supplies for use with AC dimmers are typically designed to operate from a single voltage power supply only, such as either 110 VAC or 220 VAC. However, converters for use with LEDs are typically suitable for use with so-called universal input power supplies that are intended to operate over a range of power supply voltages, such as 85-277 VAC so as to be suitable for both the US and European markets. Therefore, in order to utilize a dimmer with LEDs while maintaining conventional drive circuitry, the dimmer should preferably be adapted to operate with a range of supply voltages. This may also militate against the use of conventional AC dimmers.
WO 03/096761 assigned to Color Kinetics, Inc. discloses methods and apparatus for facilitating the use of LED-based light sources on AC power circuits that provide signals other than standard line voltages thus allowing LED-based light sources to be coupled to AC power circuits that are controlled by conventional AC dimmers, Optionally, a microprocessor-based controller may be used to provide to appropriately condition an AC signal provided by a dimmer circuit so as to provide power to one or more LEDs of the lighting unit. Thus, the microprocessor may be configured to digitally sample the dimmer output voltage and process the samples according to some predetermined criteria to determine if one or more functions need to be performed. By such means, an AC dimmer circuit may be used to adjust one or more parameters of generated light via user operation of the dimmer. The parameters of light that may be adjusted include intensity, brightness or color (e.g. hue, saturation or brightness) that may be controlled in response to dimmer operation. For example, the sampled dimmer voltage may be mapped to stored values of various control signals used to control the LED-based light source, such as duty cycles of PWM signals respectively applied to differently colored LEDs of the light source. The microprocessor may also be configured to “evaluate” the dimmer output voltage and perform one or more functions in response thereto. By such means, the microprocessor-based controller is able to sample the AC dimmer output voltage or a control signal characteristic of the degree of angle modulation (“firing angle”) and to use the resulting signal to adjust brightness of the LEDs.
However, in all embodiments thereof, the control circuitry itself is powered by the AC dimmer output voltage. As a result, when the dimmer is set too low, there is the risk that there will be insufficient voltage to power the controller. This creates a dead space of the dimmer, where the controller is shut down and the LEDs are consequently extinguished. This deficiency is acknowledged, for example, on page 19, lines 24-27, where it is stated that if the dimmer is adjusted such that the AC signal is no longer capable of providing adequate power to the drive circuitry, the light source merely ceases to produce light.
Likewise, with regard to those embodiments that use a controller to process the dimmer output, it is noted on page 26, lines 5-8 that as the overall power provided by the AC signal is reduced due to operation of a dimmer, at some point the power circuitry will be unable to provide sufficient power to the various components of the lighting unit and it will cease to generate light.
WO 03/096761 states that it provides sufficient power to the lighting unit “over a significant range of dimmer operation.” It is instructive to determine the range of dimmer operation over which the lighting unit described by WO 03/096761 remains illuminated. As shown in FIG. 6, the AC dimmer voltage is fed to a TNY266 IC switch manufactured by Power Integrations, Inc. of San Jose, Calif. USA that operates as a DC converter to produce a constant DC output voltage from a range of input AC voltages, Reference to the TNY266 Data Sheet shows that it operates over a universal voltage supply (85-265 VAC). This implies that if the AC dimmer RMS output voltage falls below 85 VAC, the TNY266 will no longer operate.
Moreover, once the dimmer voltage is increased beyond this “dead space”, the power circuitry suddenly kicks in with a correspondingly higher control voltage. Thus, in the case where the control voltage is derived from the dimmer modulation (or firing) angle and assuming that effective control by the dimmer requires adjustment of the firing angle between 0 and 90° in both AC half-cycles, the DC converter kicks in only when the minimum firing angle is reached. And if the lamp intensity is a linear function of the firing angle, this means that not only is the dimmer inactive over much of its range, but also that when it does become active the lamp will hardly be particularly dim.
This limitation is not important to WO 03/096761 since its main object is not to control the intensity of the lights but rather their color, which is varied by combining different colors of more than one light source. To this end, each light source may be independently varied in response to a common control signal. This may be done by using tables to map different PWM duty cycles for each light source and to employ a different table for each lamp. By such means, millions of colors may be generated, which may also be combined to form white light.
Thus, WO 03/096761 appears to offer a circuit for varying the colors of LED arrays over a limited range of an AC dimmer. It does not provide a circuit for varying the intensity of an LED over substantially the full range of an AC dimmer.
It is apparent that the limitations of WO 03/096761 derive from operating the power control circuitry directly from the AC dimmer output. However, there is an advantage in doing so because it ensures that at all times the AC dimmer is loaded. In normal conditions, the lamp itself loads the dimmer and this reduces lamp flicker that would otherwise ensue were the dimmer to be unloaded during part of the AC supply cycle. So it would be desirable to provide a flicker-free method and circuit for varying the intensity of an LED over substantially the full range of an AC dimmer.
WO 03/058801 in the name of the present applicant discloses a lamp transformer for use with an electronic dimmer and method for use thereof for reducing acoustic noise.
WO 06/018830 in the name of the present applicant discloses use of a controller to reconstruct suitably amended waveforms for leading and trailing edge dimmers.
The full contents of each of the above-references are incorporated herein by reference.